The Z-Series is our Sharpest, Most Durable Blade Ever! When creating these new blades, the goal was to design, develop, and manufacture a number 11 blade that is absolutely "best in class"...and we succeeded. The gold hue on the cutting edge is a zirconium nitride coating applied at the atomic level after blades are atomically sharpened. The result is the sharpest blade in the industry. The Z-Series #11 blades fits all #1 X-Acto knife handles.